The effective thermal conductivity of soil or soil-like granular materials is a key property which can significantly affect the design and performance of a buried system which must dissipate heat to, or extract heat from, its surroundings. For example, efficient operation of systems including direct buried high voltage power transmission cables, pipe grids for ground source heat pumps and oil pipe lines depends upon the transfer of heat between system components and the ground. In the foundry industry, the effective thermal conductivity of sand coating mold inserts can affect the quality of the castings obtained from the molds.
Examining the case of buried high voltage transmission cable in more detail, it can be shown that the power that can be delivered to a load depends upon the current carrying capacity (ampacity) of the cable. This ampacity is limited by the operating temperature of the cable which in turn depends upon the ability of the surrounding soil to dissipate the heat generated in the cable. Failure of the soil to sufficiently dissipate the heat can result in failure of the cable insulation. Also, at lower temperatures, the efficiency of power transmission of the cable is improved due to a lowering of the I.sup.2 R losses. The use of thermal backfill materials to surround undergound cable and the like has been suggested in order to enhance the heat dissipation of dry sand.
It has been generally accepted that while other materials, e.g., pure quartz, alumina, beryllia and the like may have higher thermal conductivities than sand and are certainly suitable substitutes, due to economic reasons, sand is preferred as the base component of such backfill compositions. While it is recognized that moist sand has a substantially high thermal conductivity (.about.3 W/m-K), once drying begins due to the heat to be dissipated, the water in the sand surrounding the heat source is evaporated and the sand dries. The dry sand, which has a thermal conductivity of only about one-tenth that of moist sand, acts essentially as a thermal insulator, inhibiting the dissipation of heat and causing reduced efficiency and possibly eventual failure of the cable. There is, therefore, a need for backfill materials having a relatively high thermal conductivity (as compared with dry sand) which is substantially independent of moisture content. These same materials, of course, may have uses in addition to that of backfill materials, such as in molds for castings.
In the past, attempts to increase the thermal conductivity of sand and sand-like materials have included mixing sand with a wax binder, mixing and firing of sand or gravel with inorganic binders such as Kaolinite, calcium carbonate, fly ash and lime. Farouki, who reported the latter experiments, concluded that for moisture contents less than about 4%, the optimum amount of inorganic binder was about 8 wt. percent. He also reported that with sand, at a dry density of about 2 gm/cm.sup.3, without binder, the thermal conductivity was 0.91 W/m-K and with 8 wt. % kaolin it was 2.0 W/m-K with little difference between binders. Mitchell, a researcher experimenting in wax as a binder alluded to the incorporation of alumina into the wax but indicated that the incorporation of additives, such as alumina into the wax, did not significantly increase the effective thermal conductivity, ke, of the composite backfill. Similarly, Cox et al, reported on cement/sand, gravel/sand and bitumen/sand mixtures as backfill materials having values of ke of no less than 0.83 W/m-K.